While I agree in most cases you don't want to rush your TH upgrades (except perhaps from TH5 to TH7, mostly skipping over TH6 to get to TH7), I disagree that you need to max everything before advancing.
Based on my experience, at TH7 I had completed maxed walls at that level (all purple) and was still upgrading buildings. The problem was that I was at a point where my builders would be tied up on lengthy projects, and I couldn't dump loot into walls because they were maxed.
This meant I sat for days with full storages. This has a two-fold negative impact:
1. With full storages you become more of a target for being attacked, so you're getting hit more and losing loot you ganked from someone else
2. Since your storages are full, you can't continue raiding. That means you're losing potentially millions in loot each day because you're sitting waiting for builders to free up.
To counter this, when you're nearing the point where your walls will be maxed at TH7, get ready to move to TH8, even if the rest of your stuff isn't maxed out. This will open up new buildings (with quicker build times since they're starting out lvl 1) as well as new walls to dump loot into.